Whether the application is macro cell or small cell base stations, RF power transistors play a critical role in determining the ultimate performance of the transmission path. Even incremental increases in RF linearity, efficiency, and gain can deliver major benefits in cost. Also, with nearly four dozen wireless bands in use globally, spanning as many bands as possible with the fewest devices has become more important than ever. Freescale has introduced three LDMOS transistors and two multi-stage integrated circuits in its Airfast family that are designed to meet these challenges, including two LDMOS power amplifiers that exceed the 50% benchmark for drain efficiency in a Doherty configuration (Figure 1).

When Freescale introduced the Airfast designation for LDMOS RF power transistors in 2012, the company’s goal was to establish performance benchmarks that all devices within the family must meet. Key among them are very high gain, linearity, and efficiency, increased bandwidth, and maintaining or reducing device footprint. These metrics are typically achieved through advances in die, matching, and package technologies, as well as device architectures.

The first devices in the family were 28-VDC LDMOS single-stage amplifiers designed for final-stage amplification. Within Airfast’s second generation of devices, the portfolio has been expanded to include multi-stage integrated amplifier ICs and will soon also include high-power GaN devices and Freescale’s line of 50-VDC LDMOS transistors.

All five devices cover multiple band allocations employed in North America and other countries throughout the world, including the 2600 MHz band widely used in China.

The A2T07H310-24S and A2T07D160W04S LDMOS transistors cover bands 5, 6, 8, 12-14, 17-20, 28 and APAC700 employed in Europe, the Americas, and elsewhere throughout the world. This has generally required multiple devices, but the A2T07H310-24S and A2T07D160W04S cover all of the bands from 728 to 960 MHz at two different RF power levels.

The two integrated amplifiers take advantage of the TO-270WBG-15 package that provides significantly greater isolation between the two Doherty amplification paths, which reduces crosstalk that degrades amplifier performance. Increasing isolation between paths would typically require increasing the physical spacing between them resulting in a larger package. However, Freescale improved isolation by up to 10 dB over the previous generation within a package that is the same size as its predecessor.

Advanced Thermal Management

Thermal management (that is, temperature control) of RF power devices is a critical requirement to ensure optimum performance and increase operating life, and quiescent current (IDQ) is a key factor. The integrated amplifiers incorporate quiescent current temperature compensation (Figure 2) that can be enabled or disabled by running a voltage source to the gate bias.

The goal of the circuit is to maintain constant quiescent current over a large temperature range, which is more difficult to achieve in multi-stage ICs. The embedded tracking circuit in the amplifiers keeps the IDQ constant for all stages regardless of changes in temperature and is capable of reducing IDQ variations to less than 5% over a 100o range.

Optimizing Video Bandwidth

As is the case with most Airfast products, the three new LDMOS devices have additional connections (Figure 3) that allow designers to increase the video bandwidth by terminating them with extra decoupling capacitance. Video bandwidth (VBW), which is synonymous with instantaneous bandwidth (IBW), is an important factor in base station amplifier design. Broadly defined, VBW or IBW is the maximum amount of spectrum a device or amplifier can process while maintaining a symmetrical and constant intermodulation product, ensuring that that clipping and increased intermodulation distortion do not occur. The instantaneous bandwidth of the amplifier typically needs to be at least three times as wide as the operational bandwidth in order to address intermodulation terms up to the ninth order.

For example, a system that transmits over an operational bandwidth of 40 MHz requires that signals (carriers) throughout this range must be linearized, the lowest and highest frequencies being the most difficult challenge. To achieve this, the VBW of the power amplifier must exceed three times the 40 MHz operational bandwidth. So if the operating bandwidth is 40 MHz, VBW should be at least 120 MHz. The goal is to ensure that IMD symmetry and flatness are achieved for at least the third-order intermodulation of the instantaneous bandwidth.

Summary

Solid-state RF power generation devices may be small, but they have an oversized role in determining overall system capability, size, cost, and reliability. They and the amplifiers from which they are built also account for a large share of a base station’s annual power consumption. Not surprisingly, it is essential that RF power devices continue to achieve greater performance with smaller physical and financial “footprints” to keep pace with the increasing cost constraints and other factors faced by wireless carriers. The new Freescale discrete transistors and integrated amplifier ICs represent a significant step forward in this direction.

About the Author:

Eric Westberg is a portfolio manager for cellular infrastructure RF products at Freescale Semiconductor. In addition to product management he has held engineering and management positions in application engineering, systems engineering, and business development in the wireless and semiconductor industries. He earned his MSEE from Arizona State University and undergraduate degrees from Wheaton College and the Illinois Institute of Technology.

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Insulated Wire Inc.

Insulated Wire Inc. is pleased to announce that following the retirement of a French delegate, IW Microwave Products Division President, John Morelli, has been appointed as the Secretary for IEC/SC46F, a subcommittee of IEC/TC46. IEC/SC46F is responsible for RF and Microwave Connectors and Components International Standardization.

Mr. Morelli, who is sponsored by Insulated Wire, brings significant RF/Microwave connector and component experience to the position, and we congratulate him on his new appointment. Mr. Morelli had been serving as the chief Technical Advisor for the US National Committee’s Technical Advisory Group to IEC/SC46F.

The US National Committee of the IEC is a fully integrated entity of the American National Standards Institute (ANSI).